This invention relates, in general, to a method and apparatus for making glass objects from glass feedstock in which the control of a dimension of the glass object is greatly improved. This invention relates particularly to the processing of core preforms to form optical waveguide cane from which overclad preforms can be produced for drawing into optical waveguide fibers. This invention also relates to the manufacture of optical waveguide components, such as couplers and planar optical waveguides, which require dimensional control during manufacture.
In the manufacture of optical waveguide, fiber, one process which is known involves the manufacture of a core preform from which optical waveguide cane is drawn. The optical waveguide cane is then overclad to form an overclad preform which is then drawn into optical waveguide fiber. This "two-step" process has the advantage of better control of the refractive index profile of the resulting fiber and of more efficient manufacturing which leads to lower costs.
A critical parameter to control, when drawing cane from a core preform, is the diameter of the cane. Variations in the diameter of the drawn cane can result in changes in the core-clad ratio of an optical fiber drawn from an overclad preform produced from such cane. These variations in the core-clad ratio will degrade the transmission characteristics of the resulting optical fiber.
A typical apparatus for drawing cane from a core preform is shown in FIG. 1. A blank feed mechanism 1 lowers a blank 5 into a furnace 2. The furnace 2 heats an end of blank 5 to its softening temperature. A cane drawing mechanism 4 draws cane 6 from the softened end of blank 5. The diameter of cane 6 is measured by a measuring device 3. The drawing rate is controlled by a computing device 8 to achieve cane with a predetermined diameter. Measuring device 3 is generally a non-contact, optical measurement to avoid damage to the pristine surface of the cane as it is drawn.
As blank 5 is softened, a cone-like root section 7 is formed from which cane 6 is drawn. The length of this cone-like root is a function of the length of the hot zone of furnace 2, with cone-like root section 7 being longer for furnaces with longer hot zones.
Although the cane diameter is generally determined by the drawing speed of mechanism 4, disturbances in the diameter of the drawn cane may be caused, for example, by variations in the physical characteristics of blank 5 and changes in the ambient conditions surrounding the cane drawing apparatus. Because of the relatively long distance between measuring device 3 and cone-like root section 7 where the cane is formed and the relatively slow speed at which the cane is drawn, the control of the diameter of cane 5 is characterized by a significant deadtime between the actual variation in the diameter of cane 6 and its detection at the measuring device 3.
In the past, changes to the drawing speed of the cane, V.sub.c, were determined by a proportional-integral (PI) control algorithm, based on the error between the measured diameter and the desired diameter or set point. The PI algorithm can be expressed as: ##EQU1## where EQU E=OD.sub.sp -OD (2)
OD.sub.sp =diameter set point PA1 OD=2*R.sub.c =measured outside diameter PA1 R.sub.c =cane radius PA1 V.sub.c (0)=initial drawing speed PA1 P=proportional control gain PA1 .tau..sub.I =integral reset time PA1 t=time PA1 h=control interval PA1 I=integral control gain
When implemented by a computer, the PI control algorithm in equation (1) above is realized in the discrete time domain as
V.sub.c (t)=V.sub.c (t-h)+P[E(t)-E(t-h)]+IhE(t) (3)
where
It is sometimes beneficial to smooth the measured diameter, OD, by a filter that reduces the impact of high frequency disturbances. That filter may include, for example, an N-point moving average to generate a filtered outside diameter, OD.sub.f, which is used in the PI control algorithm error of equation (2): ##EQU2## Equation (2) is then rewritten as: EQU E(t)=OD.sub.sp -OD.sub.f (t) (5)
As the economic benefits of larger blanks force the use of larger furnaces in the redraw process, the deadtime discussed above increases, posing severe problems for this conventional PI control algorithm which degrade the control of the outside diameter of a drawn cane. For example, when a variation occurs in the outside diameter of the preform, a corresponding change will occur in the outside diameter of the cane being drawn from the preform. This change in diameter will be measured by measuring device 3 and the diameter control loop will cause a change in the drawing speed of mechanism 4. The effect of this change on the outside diameter of the cane will not be measured by measuring device 3 until a length of time equal to the deadtime has expired. Because the control interval, h, is smaller than the deadtime, the diameter control loop will continue to adjust the drawing speed of mechanism 4 based on the measured values of the outside diameter of the cane. If the control interval is equal to or greater than the deadtime, the diameter control loop will not be responsive enough to short term disturbances.
Loxley et al. U.S. Pat. No. 3,652,248 discloses a process for drawing microbore quartz glass tubing, the diameter of said tubing being about 0.1 inch or less, wherein the stability of the outer diameter is enhanced by applying cooling jets of air or inert gas about 1 to 2 inches below a flame heat source used to soften the glass rod feedstock. These cooling jets stabilize the point at which the drawn tubing solidifies to a constant diameter. Adjustments to the fuel mixture to the flame heat source and the cooling gas are made by "skilled operators" based on observations of a thickness gauge reading to obtain the desired final diameter. col. 6, lines 63-73. There is no disclosure or suggestion in Loxley et al. of any control algorithm on which changes to these flow rates are based.
Mellor et al. U.S. Pat. No. 4,076,512 discloses a method for producing a clad glass rod from which optical fiber is drawn. These rods have diameters between 1.5 mm and 8 mm with the diameter controlled to within .+-.5%. Mellor et al. discloses the use of a "high gain closed loop system" utilizing "a proportional plus integral controller" to regulate the speed of drawing the glass rod based on the measured diameter of the formed rod. col. 2, lines 46-68. There is no disclosure or suggestion in Mellor et al. of any compensation for the deadtime between a change in the drawing rate and the measurement of the change in outside diameter of the glass rod corresponding to the change in drawing rate.
Clark et al. U.S. Pat. No. 4,631,079 discloses a method for drawing glass rods wherein a portion of a glass rod is heated to a first temperature high enough for reflow of the glass, the heated portion is necked down to approximately the desired diameter, the rod is cooled, the necked down portion is progressively reheated to a temperature less than the first temperature but sufficient for reflow, and the rod is stretched during the progressive reheating to the desired final diameter. col. 2, lines 16-27. By reheating the rod at a lower temperature, the viscosity of the glass is increased, and diameter fluctuations in the final drawn rod are reduced because the response of the rod to stretching conditions is dampened by the higher viscosity. Clark et al. does not disclose or suggest any measurement of the diameter of the glass rod during the heating or reheating steps described above. There is also no disclosure in Clark et al. of any control system for regulating stretching conditions to control the diameter of the drawn rod.